In recent years, in a cellular mobile communication system, with a trend toward multimedia information, transmission of large-volume data, such as still image data and motion image data, as well as sound data has been generalized. In order to realize transmission of large-volume data, studies on a technique for realizing a high transmission rate using a high-frequency radio band has been popularly conducted.
However, when the high-frequency radio band is used, while a high transmission rate can be expected at a short range, attenuation due to a transmission distance increases at a longer distance. Accordingly, when a mobile communication system using a high-frequency radio band is practically operated, since the coverage area of a radio communication base station apparatus (hereinafter, abbreviated as “base station”) becomes small, it is necessary to establish more base stations. The establishment of the base stations requires reasonable costs. Therefore, there is strong demand for a technique for realizing a communication service using a high-frequency radio band while suppressing an increase in the number of base stations.
For this demand, in order to expand the coverage area of each base station, a relay transmission technique in which a radio communication relay station apparatus (hereinafter, abbreviated as “relay station”) is provided between a base station and a radio communication mobile station apparatus (hereinafter, abbreviated as “mobile station”), and communication between the base station and the mobile station is performed through the relay station has been studied. If the relay technique is used, a mobile station which cannot perform direct communication with a base station can perform communication through a relay station.
In regard to an LTE-A (Long Term Evolution Advanced) system in which the introduction of the above-described relay technique has been studied, from the viewpoint of smooth transition from LTE (Long Term Evolution) and coexistence with LTE, there is demand for maintaining compatibility with LTE. For this reason, in regard to the relay technique, there is demand for mutual compatibility with LTE. In the LTE-A system, in order to attain compatibility with LTE, at the time of transmission from a base station to a relay station in a downlink (hereinafter, referred to as “DL”), the setting of an MBSFN (MBMS Single Frequency Network) subframe has been studied. The term “MBSFN subframe” is a subframe which is defined so as to transmit MBMS (Multimedia Broadcast Multicast Service) data. An LTE terminal has a specification for an operation so as not to use a reference signal in an MBSFN subframe. Accordingly, in LTE-A, a method in which an access link subframe overlapping a backhaul link subframe used for communication between the relay station and the base station is set in an MBSFN subframe has been proposed. With this proposal, it is possible to avoid the LTE terminal from erroneously detecting the reference signal.
The communication between the base station and the mobile station through the relay station is performed by time-division relay (called TD relay or Type 1 relay). Backhaul communication (that is, communication between the base station and the relay station) and relay access link communication (that is, communication between the relay station and the terminal) are divided on the time axis, thereby dividing the transmitting time and the receiving time of the relay station. Accordingly, the relay station can perform relay without being affected by wraparound between a transmitting antenna and a receiving antenna.
FIG. 1 shows an example of an allocation state of control signals and data for each station of a base station, a relay station, and a mobile station when a subframe of an LTE system is used. As shown in FIG. 1, in the LTE system, a downlink control signal which is transmitted or received in each station is arranged in a PDCCH (Physical Downlink Control Channel) region of a head portion of the subframe. That is, both the base station and the relay station transmit the control signal in the PDCCH region of the head portion of the subframe. When focusing on the relay station, since the downlink control signal should be transmitted to the mobile station even in the MBSFN subframe, the relay station transmits the control signal to the mobile station and is thereafter switched to receiving processing, thereby preparing for receiving a signal transmitted from the base station. However, since the base station also transmits the downlink control signal intended for the relay station at the timing at which the relay station transmits the downlink control signal to the mobile station, the relay station cannot receive the downlink control signal transmitted from the base station. In order to avoid this inconvenience, in the LTE-A, providing a region for arranging the downlink control signal for the relay station (R-PDCCH (Relay PDCCH) region) in a data region has been studied.
In LTE, a DL grant instructing DL data allocation and a UL grant instructing UL data allocation are included in the PDCCH. With the DL grant, a resource in a subframe in which the DL grant is transmitted is allocated to the mobile station. In regard to the UL grant, in an FDD system, with the UL grant, a resource in a target subframe next to four subframes after a subframe in which the UL grant is transmitted is allocated to the mobile station. In a TDD system, with the UL grant, a resource in a target subframe next to four or more subframes after a subframe in which the UL grant is transmitted is allocated to the mobile station. In the TDD system, how many subframes are next to a subframe to which a resource is allocated by the UL grant after a subframe in which the UL grant is transmitted is determined in accordance with a pattern in which an uplink and a downlink are time-divided (hereinafter, referred to as “UL/DL configuration pattern”).
In LTE-A, including a DL grant and a UL grant in an R-PDCCH has been studied. In the R-PDCCH, arranging the DL grant in a first slot and the UL grant in a second slot has been studied (see NPLs 1 and 2). In this way, if the DL grant is arranged only in the first slot, the decoding delay of the DL grant is reduced, thereby preparing for ACK/NACK transmission for DL data (in FDD, transmission next to four subframes after reception of the DL grant).
As shown in FIG. 2, a method in which a resource block of a physical layer (PRB) in which an R-PDCCH region is provided differs between the relay stations has been studied. The relay station performs blind decoding on the downlink control signal, which is transmitted from the base station using the R-PDCCH region in this way, in a resource region instructed from the base station by higher layer signaling, and the downlink control signal intended for the relay station is found.